Polymeric films such as opaque polymeric films are used in many applications including packaging or labeling materials, battery separator, filter, membrane, etc. Various organic and inorganic cavitating agents have been used in the production of opaque films. For example, organic materials such as polybutylene terephthalate (“PBT”), polyethylene terephthalate (“PET”), poly(ethylene 2,6-napthalate) (“PEN”), polycarbonate (“PC”), nylons, cross-linked polystyrene, syndiotactic polystyrene, acetals, acrylic resins, polyacrylate, poly(N-vinylcarbozole), polyvinylcyclohexane, polyvinyl chloride, polyacrylonitrile; and inorganic materials such as glass, metal, and ceramic have been described and/or claimed as cavitating agents, for example, in U.S. Pat. Nos. 4,632,869; 5,134,173; 5,573,717; 6,048,608; 5,866,246; and 6,528,155. Other cavitating agents include polymorphic crystals of polymers such as β-polypropylene, as disclosed in U.S. Pat. No. 6,828,019.
U.S. Publication No. 2009/0191388 discloses a biaxially oriented film that comprises polypropylene and either polytrimethylene terephthalate or polymethylpentene as a cavitating agent.
The presence of a cavitating agent in a layer of a film structure during orientation of the film structure induces voids within the polymeric material of the layer. The voids scatter light thereby causing the film structure to be translucent. Thus, a film containing such voids may be opaque. The voids may also impart the film structure with enhanced permeability to gas and moisture. An opaque polymeric film may be porous or nonporous.
Polar polymers, either crystalline such as PET or amorphous such as PC, are often used as a cavitating agent for polyolefin films because they create a high interfacial tension with the nonpolar polyolefin matrix, which promotes the formation of a well-dispersed multiphase morphology in-situ during extrusion that is suitable for voiding in the orientation process. However, polar polymers are hydrophilic, thus sensitive in general to hydrolytic breakdown, and readily degrade into lower molecular weight species when they are extruded with nonpolar polymers as a blend component.
The lower molecular weight species such produced are known to migrate to and build up on the surface of the processing apparatus, e.g., screws, barrel walls, melt pipes, screen packs, dies, etc. The build-up often sloughs off the metal surfaces and passes into the film as sizable deposits of hard, eggshell-type impurities. These impurities are one of the primary causes of film splitting during orientation and optical non-uniformity of films such as opaque film. In order to mitigate the hydrolytic breakdown, the polar cavitating agent polymer is generally dried thoroughly to a moisture level as low as 200 wppm or less before being extruded as a film.
In addition, film scraps and edge trim containing polar cavitating agent (e.g., PBT) polymers often cannot be recycled, even in small quantities, because of the high reactivity of the polar component with polar additives and catalysis residues. Thus, clean polyolefins, containing the lowest possible concentration of catalyst residues and impurities, are often used in the film manufacture to suppress unwanted reactions and degradation of the polar cavitating agent polymer.
A need still exists for a cavitating agent that allows for improved film opacity and machinability. Preferably, such a cavitating agent produces little or less extrusion plate-out or build-up on the processing apparatus; has little or less interaction with catalyst residues, inorganic fillers such as TiO2, or other additives contained within the film, thus allowing for the recycling of film scraps and edge trim; and produces uniform voiding across the film width, reducing processing issues such as split, bumps, sagging, or curvature.